It is well known to evaluate the condition of a patient's heart by monitoring electrical signals produced by the heart and storing and/or displaying a representation of these continuously monitored heart activity indicating signals on a suitable medium, such as by means of a strip chart recorder or a cathode ray tube. The recording or display takes the form of a waveform describing heart electrical output versus time which is known as an electrocardiogram, sometimes abbreviated ECG.
The electrical signals representing heart activity are conventionally picked up by a combination of three electrodes. Each electrode is applied to a portion of the patient's body through an interface of a conductive gel to improve conductivity and to lessen electrical artifacts which occur in the course of an ECG study. Each electrode is connected by an electrical cable to an electronic unit which is sometimes called a portable acquisition unit.
The purpose of the portable acquisition unit and its amplifier circuitry is to condition the electrical heart activity indicating signals of physiological origin so that they can be transmitted from the body to the heart activity display and recording apparatus.
There are two basic problems associated with the extraction of the heart activity indicating information. First, the signal amplitude is very low coming from a fairly high source impedance. Secondly, the heart activity indicating signals exist in the presence of significant extraneous "noise" signals. The noise signals originate from low frequency and high amplitude artifacts. These artifacts are created by the electrode interface used to sense the physiological signals and also from slightly higher frequency (60, 120 Hz.) electromagnetic interference, as well as from physical phenomena relating to the electronic circuit elements.
In a conventional ECG system, a first electrode is attached to the patient's side, a second electrode is attached near the patient's sternum, and a third, or reference, electrode is attached to the patient's right leg or abdomen. The first and second electrodes produce the heart activity signals.
Despite the use of the conductive gel, the electrical artifacts still occur superimposed on the desirable heart activity indicating signals. These artifacts often have an amplitude much greater than that of the heart activity indicating signals, and consequently, when they occur, they swamp out the useful information contained in the heart activity indicating signals whose monitoring is desired. More specifically, the heart activity indicating signals are usually in the one millivolt range, while the artifacts are in the 20-30 millivolt range, and can run as high as about 200 millivolts.
Typically, the circuitry of the portable acquisition unit has a dynamic range which enables the unit to accommodate incoming heart activity indicating signals up to about 5 millivolts. Any signals greater than that value, including artifact signals, will drive the electronic circuitry of the portable acquisition unit into saturation, and will thus obliterate any desirable information component of heart-generated electrical signals being received at the time of occurrence of the artifact. The problem of these artifacts is exacerbated by the fact that the portable acquisition unit typically includes a high gain amplifier for amplifying the heart activity indicating signals to render them suitable for driving the strip chart recorder or for actuating a cathode ray tube (CRT) display which provides a real time display of the waveform of the heart activity indicating signals.
Common forms of portable acquisition units have included a differential preamplifier. The differential preamplifier receives electrical signals from the cables connected to the first and second electrodes described above. The differential preamplifier produces a continuous real time output signal which is a function of the difference between the signals on the first and second electrodes. The preamplifier stage also amplifies this difference signal by a small gain.
The output of the differential preamplifier stage is directed to other downstream circuitry of the portable acquisition unit. The downstream circuitry provides additional or main amplification, to raise the total gain to something in the neighborhood of 1,000. The downstream circuitry can also provide for other forms of desirable signal processing to render the signal more appropriate for driving the real time display or the strip chart recorder or any other form of signal storage device.
Circuitry has been provided to reduce the undesirable effects of electrical artifacts superimposed on the desirable heart activity indicating signals. One such technique has been to "AC couple" the differential preamplifier stage to the downstream circuitry. This has been done by interposing an AC coupling capacitor in series in the path of the output signal from the preamplifier. In such an embodiment, a resistor is added between the downstream terminal of the capacitor and ground.
The capacitor blocks transmission of DC or very low frequency signals before they reach the downstream circuitry containing the main amplifier. Since the artifacts often take the form of a sharp change in overall DC level, or are of very low frequency, the capacitor is effective in blocking out the electrical artifacts which make their way through the preamplifier stage.
A significant problem with this approach is that, during the time that the blocking capacitor and its associated resistor are responding to transients caused by the electrical artifacts in order to block those transients, they are also blocking the transmission of the useful hard activity indicating signals. In order to block the undesirable transients, the capacitor responds by storing up electrical charge resulting therefrom, and by subsequently and relatively slowly discharging that extra charge through the associated resistor to ground. In order to be effective, the capacitance of the capacitor and the value of the associated resistor must be chosen such that the time constant of this RC combination is on the order of 3 seconds. This means that, when a transient occurs and is blocked and drained off by the RC combination, the useful information borne by the heart activity indicating signals is also effectively blocked from the downstream circuitry for a period at least equal to the time constant of the RC combination, and often, in practice, for a much longer time than that, i.e., on the order of 10 or 11 seconds.
Thus, when AC coupling is employed to block transients resulting from undesirable electrical artifacts, there occurs a time window associated with each artifact during which the desirable heart activity indicating information is not transmitted either. This creates a very undesirable discontinuity in the receipt and recording of heart activity indicating information.
It is a general object of this invention to provide an ECG system having circuitry which, while effectively blocking electrical transients resulting from artifacts, nevertheless maximizes the continuity of receipt of desirable hear activity indicating information.